Objective: This systematic review aims to identify, critically appraise and summarize the results of studies examining the relationship between the vaginal microbiota and preterm birth (PTB). Methods: We searched the electronic databases Medline, EMBASE and the Cochrane Controlled Register of Trials for studies in any language reporting on vaginal microbiota and PTB published from 1990 to November 29th, 2017. We included any study that performed lower genital tract microbiota assessment in asymptomatic pregnant women and reported on spontaneous preterm birth, with either intact or ruptured membranes. Results: The search strategy yielded 2171 unique citations, of which nine studies were eligible for inclusion in this review. In six studies an association was found between the composition of the vaginal microbiota and PTB, but findings differed between subgroups, ethnicities and degree of risk of PTB. In three studies no association was found. Two of these studies found a significant difference in richness and Shannon diversity between term and PTB. Conclusions: We have demonstrated that there is a paucity of molecular based, culture-independent studies that analyse the relationship between the vaginal microbiota and PTB as an outcome. The heterogeneity precluded a meta-analysis. Studies provide contradictory evidence and the quality of the clinical information in the studies is poor. To improve quality of future studies we have provided a database of essential and desirable items of quality that are method and topic specific.